Transition from one background to another as object moves in front of camera
The main thing is that the plane wing does not provide the full mask alpha that you require to do the transition. You need to create a second mask (either with an actual mask object in the compositor, or by rendering a second object on a separate render layer) that you can add to the alpha from the plane wing.

Please note I've made the "wing" object more irregular shaped so that the effect is more obvious.